Heck et al. describe rheology modified TPE compositions in WO 98/32795. The rheology modification can be induced by various means including peroxides and radiation. The compositions of Heck et al. are said to exhibit a combination of four properties: shear thinning index (STI), melt strength (MS), solidification temperature (ST) and upper service temperature (UST). Heck et al. rheology modifies the compositions using a peroxide alone. Heck et al. teach the polymer compositions must have an STI of at least 20, preferably at least 25, more preferably at least 30, and still more preferably at least 35. While these compositions may be useful in some applications such as automotive parts and boot shafts, improved compositions are needed for extrusion, molding, and calendaring operations, especially high speed operations.
Walton describes in WO 02/24803 rheology modified compositions for calendaring and thermoforming operations. The compositions are described broadly as comprising a peroxide and coagent in a 1:10 to 10:1 ratio, although in the examples, rheology modification is accomplished by use of a peroxide and coagent in an approximately 1:2 to 2:1 ratio. (One skilled in the art understands that ratios are defined using whole numbers only.) The rheology modified compositions of Walton exhibit a melt toughness of at least about 600 cN•mm/s, a true ultimate tensile strength at 140° C. of at least about 3 MPa and an elongation to break at 140° C. of at least about 400%. The Walton compositions can also have measureable gels which are undesirable for many extrusion and other molding processes. While these compositions may be useful for some calendaring operations, improved compositions are needed for extrusion, molding, and calendaring operations where a smooth surface, low odor, collapse resistance, and high productions line speeds are desired.
Compositions having a smooth surface are desirable for extrusion, molding and calendaring. Shape retention or collapse resistance is particularly important for profile extrusion. Compositions having low odor are desired for all operations, including extrusion, molding and calendaring, and those which are capable of being extruded at high line speeds are especially desirable for extrusion and blow molding operations. A smooth surface is aesthetically pleasing and sometimes required to obtain good sealing properties, shape retention is needed to prevent collapse of the extruded profile after exiting the die, and low odor is desirable to minimize worker exposure to chemical residuals during the manufacturing process. High line speeds translate into improved productivity for the manufacturer.
Surprisingly, it has been discovered that by rheology modifying TPE compositions with low levels of peroxide and high levels of coagent, relative to peroxide levels, compositions are produced which can be extruded at very high line speeds and provide articles with excellent surface and odor characteristics.